The invention relates to predistortion in audio circuits, and particularly to a circuit for optimizing third harmonic distortion in an analog recording by over-recording signal peaks to compensate for tape saturation effects.
As commonly known in the analog recording field, magnetic recordings are adversely affected by harmonic and inter-modulation distortion, especially at a high recording level wherein tape saturation effects are of major concern. Equally well known are non-linear circuit techniques for properly predistorting the recording signal to reduce the harmonic distortion.
One such technique for properly predistorting the recording signal, is to calculate several points on a X.sup.3 curve, i.e., several different amplitudes weighted towards the levels of saturation on the tape where predistortion particularly is needed. Then an amplifier circuit is designed to provide diode break points such that the gain is increased at the output of the amplifier when the recording signal reaches a selected point, increases again when it reaches a second selected point, etc. This provides accordingly a piece-wise linear approximation of the curve X.sup.3. Such a technique is relatively inexpensive, requiring only one-percent resistors to insure that the break points are properly located. However, such technique is also very temperature sensitive, since there are numerous diodes strung together and the gains can vary due to variations in the selected break point values, caused in turn by temperature changes.
A more sophisticated technique for providing proper predistortion of a recording signal is through the use of four-quadrant multipliers. The multipliers are disposed in series, wherein the first multiplier provides the square of the input signal, while the second multiplier provides for multiplication of the first multiplier output with the input signal. The result is a predistortion error signal corresponding to the cube of the input signal; i.e., an output signal corresponding to the third harmonic distortion.
However, each four-quadrant multiplier has two offsets which need be adjusted, wherein all four adjustments are interdependent. Thus, what theoretically appears to be a relatively straight-forward adjustment is, in practice, relatively difficult due to the interdependence of the multiple offset adjustments.
In addition, four-quadrant multipliers inherently tend to be very noisy devices. Thus, the devices tend to add noise to the predistortion signal which may further deteriorate the signal-to-noise ratio of the recording signal. As a result although the signal-to-noise is not tape limited, it is instead limited by the predistortion circuit.
A further disadvantage of the four-quadrant multiplier predistortion circuit is that such devices are relatively expensive.
The invention overcomes the shortcomings of the typical predistortion circuits of previous mention, by providing a relatively inexpensive predistortion circuit which has only one offset adjustment and which generates a truly linear predistortion signal. Further, the circuit is relatively independent of temperature variations.
To this end, the input signal to the predistorted circuit is fed to a full wave rectifier as well as to an operational transconductance amplifier. The latter amplifier has a gain stage whose gain is set by a control current. The amplifier output is fed to a second operational transconductance amplifier, which thence supplies the output predistortion signal corresponding to the third harmonic distortion. The output of the full wave rectifier comprises the absolute value of the input signal and is supplied to the gain control input of each of the two amplifiers. Thus the output of the first amplifier is the abosolute value of the input signal multiplied by the input signal and its sign, i.e., is the square of the input signal times the sign of the input signal. The output of the second amplifier comprises the square of the input signal multiplied again by the absolute value of the input signal, which corresponds to the input signal cubed. The cubed output signal from the second amplifier then is added to the initial input signal as an error term, to provide the predistortion recording signal to the head. A single offset adjustment is provided at one of the amplifiers.
Accordingly, it is an object of the invention to improve the third harmonic distortion in analog recordings by over recording signal peaks to compensate for tape saturation.
Another object is to provide a predistortion error signal with truly linear functionality and a minimum of adjustments.
A further object is to provide for predistortion signal generation via an inexpensive circuit which further is relatively temperature independent.
A yet further object is to generate a predistortion signal via first squaring and then cubing the input signal.